Stratigraphic Controls on Reservoir Properties, Cretaceous Niobrara Formation, DJ Basin, Colorado

Deacon, Marshall; McDonough, Katie-Joe; Brinton, Lise; Friedman, Scott; Dunn, Joe; Lieber, Robert

The Cretaceous Niobrara Formation produces oil and gas from the well-studied chalk/marl cycles deposited in the Western Interior Cretaceous (WIS) Seaway. Despite relatively easy subsurface correlation, the problem of understanding reservoir property distribution persists. We evaluate the correspondence between depositional facies, cycle stacking pattern and reservoir characteristics such as porosity, permeability, wetting phase, and brittleness.

This work follows on detailed facies delineation resulting from our analysis of twelve slabbed cores through the Niobrara interval in the DJ Basin. Facies characteristics were defined conventionally (lithology, sedimentary structures, ichnofabrics), enhanced with petrographic analysis and SEM imaging. Facies and sedimentologic data were captured quantitatively using WellCAD core description software. We then integrated facies and cycle data at two scales: 1) the basin scale, by detailed correlation and mapping the extent of facies tracts within chronostratigraphic units, and 2) the reservoir scale, by processing well logs with a fully integrated petrophysical model, calibrated with the depositional facies. Both raw and processed log data crossplots yielded reasonable relationships with depositional facies, although some features remain underdetermined using conventional log data suites.

Regionally, several sequences and their associated systems tracts were defined. Within sequences, lateral shifts in facies tracts occur at cycle boundaries, corresponding to sea level- and climate-driven changes in ocean circulation pattern. Increasingly open marine conditions ("transgression") resulted in widespread ‘chalking' cycles, whereas increasingly restricted marine conditions ("regression") indicated by increasing clay-rich fluvial detritus resulted in regional and local ‘marling' cycles. Aggradational cycle turnarounds are associated with two important facies - the most favorable reservoir facies and highest source rock facies. At chalking-to-marling turnarounds, reservoir facies occur interbedded with thin organic-rich zones having a crinkled, microbial mat-like aspect.

Integration of this facies and sequence based model resulted in an improved understanding of the controls and predictability of Niobrara reservoir performance.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013